The present invention relates a vibrator and ultrasonic motor employing the same, and more particularly to a vibrator for exciting substantially elliptical motion and an ultrasonic motor employing such a vibrator.
Ultrasonic motors operate by holding a moving element under a prescribed pressure against a vibrator which excites substantially elliptical motion and driving the moving element under frictional forces between material points which make substantially elliptical motion and moving element. Since a standing-wave-type vibrator is easily constructed to achieve highly efficient vibration, a standing-wave-type ultrasonic motor is higher in efficiency and output than a travelling-wave-type ultrasonic motor.
Vibrators for use in conventional standing-wave-type ultrasonic motors employ a mechanical resonant system having a natural vibration mode which effects unidirectional vibration at the surface of contact with electromechanical transducer and elliptical vibration at the surface of contact with a moving element, so that substantially elliptical motion can be excited highly efficiently. Therefore, there are imposed certain limitations on the configuration and size of the ultrasonic vibrator, and vibrators of desired shape and size cannot be produced. Since the single natural vibration mode is employed, the ratio of the longer to shorter diameters of the substantially elliptical motion that is excited is solely determined by the shape or the like of the vibrator.
The standing-wave-type ultrasonic motor with the above vibrator incorporated can achieve highly efficient operation only in a very limited range. The standing-wave-type ultrasonic motor is poor in its ability to accommodate load fluctuations, cannot be reversed in rotation, and hence has found only a limited range of applications.
There has been an inhouse proposal (Japanese patent Application No. 62-175043) which proposes vibrator which can highly efficiently generate substantially elliptical motion having a large amplitude and can easily control the path of such substantially elliptical motion. The disclosed vibrator produces substantially elliptical motion which is a combination of circumferential shearing vibration and axial flexural vibration which are excited in a planar resonant member.
Circumferential shearing vibration can easily be excited by a method as disclosed in Japanese Laid-Open Patent Publication No. 61-120678, for example.
When circumferential shearing vibration is excited in a planar resonant member, however, since strains are concentrated on the inner circumferential edge thereof, it is difficult to fix the planar resonant member and effect impedance matching, and the boundary conditions tend to become unstable. Therefore, the ultrasonic motor does not operate stably.
According to the above disclosed exciting method, the inner circumferential portion of the planar resonant member cannot strictly be regarded as a fixed end in an ultrasonic range. Therefore, difficulty is experienced in designing an ultrasonic motor using the exciting method.
SUMMARY OF THE INVENTION
In view of the aforesaid defects of the conventional vibrators and ultrasonic motors, it is an object of the present invention to provide a vibrator which can stably excite circumferential shearing vibration and can be designed with ease, and an ultrasonic motor which employs such a vibrator.
According to the present invention, there is provided a vibrator comprising a resonant member having a central hole defined therein and a circumferential portion, first exciting means for exciting circumferential shearing vibration in the resonant member such that the circumferential portion and the hole are positioned at antinodes of the circumferential shearing vibration about the hole, and second exciting means for exciting vertical vibration or flexural vibration in the resonant member in a direction different from the direction in which the circumferential shearing vibration is excited by the first exciting means.
With this structure, an ultrasonic vibrator has a moving element held against the resonant member in the vibrator. The resonant member in the vibrator is caused to excite the vibrations by the first and second exciting means. In the shearing vibration, the resonant member resonates while the circumferential portion and the hole thereof are serving as free ends. The vibrations in the two different directions are combined in the resonant member, producing substantially elliptical vibration.
The moving element in the ultrasonic motor is rotated by frictional forces produced between the moving element and the surface of the resonant member which generates the substantially elliptical vibration.
The above and other objects, features and advantages of the present invention Will become more apparent from the following description when taken in conjunction with the accompanying drawings in Which a preferred embodiment of the present invention is shown by way of illustrative example.